1. Field of the Invention
The present invention relates to a driving mechanism for the upper looper in overlock sewing machines, of the type comprising: a drive shaft rotatably mounted to the bed of a sewing machine and operated with an angular oscillatory movement about its own axis; a driving arm fastened to the drive shaft and extending therefrom in a substantially radial direction; a support arm pivoted to the driving arm at the lower part thereof and fixedly supporting said upper looper; a bearing pin connected to the machine bed and disposed sideways of the support arm, on the opposite side relative to the drive shaft; an auxiliary arm rotatably connected to the bearing pin, extending from said pin in a substantially radial direction and having its free end pivotally mounted to the support arm.
2. Description of the Related Art
It is known that in overlock sewing machines the upper looper performs a reciprocating motion following a path which extends on either side of the edges of the needle plate and the workpiece. At the bottom dead center of its stroke the upper looper engages the thread carried by the lower looper and then jumps over the needle plate and workpiece. In this manner the thread carried by the upper looper is arranged so that it is engaged by the needle or needles above the workpiece itself.
From the foregoing it appears that the upper looper must take a very precise path, dictated by various parameters among which it is particularly important the thickness of the workpiece, the width that the line of stitching must have and the number of needles provided for carrying out the stitching.
Presently the displacements of the upper looper according to the desired path can take place by means of various driving mechanisms.
According to one known driving mechanism the upper looper must be fastened to a support arm the lower end of which is pivotally mounted to a driving arm which, through a shaft rotating about its own axis, receives an oscillatory movement in a substantially vertical direction. The support arm is also fastened, close to its upper end, to an auxiliary arm extending in an opposite direction relative to the driving arm, from a respective bearing pin mounted to the sewing machine bed. Due to the connection created by the auxiliary arm, when the driving arm causes the support arm and the looper to rise, the upper part of the support arm and therefore the looper tip is made to rotate about the axis of the bearing pin.
The combination of movements transmitted by the driving arm and the auxiliary arm respectively, causes the looper tip to move according to a path of the type shown at A in FIG. 1. Also diagrammatically shown in said figure is the needle plate of the machine, seen in cross-section and identified by 5 and the path taken by the needle, identified by 7a, as well as that taken by the lower looper and identified by 8a.
Although the above described deriving mechanism has proved to be capable of performing the functions it has been designed for, it has however an important drawback in that it does not allow the looper path to be altered when the sewing machine must be used with a workpiece having a different thickness or when it is necessary to perform lines of stitching having a different width and/or using a different number of needles as compared to the preceding sewing. Therefore until now the sewing machines using this type of driving mechanism could only be used for carrying out a specific type of working.
In order to achieve a greater versatility of use in the field of these sewing machines a driving mechanism has been designed in which the support arm of the upper looper is slidably engaged through a cylindrical element which in turn is accommodated, rotatably about a horizontal axis, within an eccentric sleeve supported by the machine bed. The support arm is engaged in such a manner that, together with the vertical displacements transmitted by the driving arm, it undergoes a rotation according to the axis of the cylindrical element, so as to guide the upper looper according to a path identified by B in FIG. 1. This path can be altered by modifying the angular orientation of the eccentric sleeve about its own axis. In fact, by rotating the eccentric sleeve, it is possible to achieve a displacement in the axis of rotation of the cylindrical element through which the support arm is guided.
It will be understood, however, that when the eccentric sleeve is acted upon in order to modify the path of the upper looper it is also necessary to carry out further long operations in order to retime the lower looper and the needle or needles relative to the upper looper. This is substantially due to the fact that when the path is modified it undergoes great displacements at its bottom dead center where the upper looper must slightly touch the lower looper in order to engage the thread of said lower looper.
In addition it has been found that the slidable engagement of the support arm in the cylindrical element often gives rise to problems concerning lubrication, wear and noise.
Driving mechanisms have been also accomplished in which the cylindrical element engaging the support arm of the looper is rotatably housed within a block in turn fastened to the machine bed by means of threaded elements operating through respective elongated holes horizontally formed in said block. By unloosing said threaded elements it is possible to horizontally shift the block and, consequently, the axis of rotation of the cylindrical element. In addition, said elongated holes are disposed in such a manner that the shifting of said axis of rotation is also possible in a vertical direction, if said block is dismantled and then mounted again in a 180.degree. overturned condition.
Due to this solution, in which the looper takes the path identified by "C" in FIG. 1, it is possible to further increase the versatility of use of the sewing machine.
On the other hand, however the adjustment of the looper path is rather difficult and unpractical.
In addition, this driving mechanism has all drawbacks described above with reference to the driving mechanism using the eccentric sleeve, as the movement of the upper looper substantially takes place in the same manner.